Roy Condrey's journey to becoming an expert on murder houses began with a dying air conditioner.

After he replaced a faulty AC unit at a rental property he owns in Irmo, S.C., the tenant told him she thought the house was haunted. On a whim, he searched the Internet for a website that would reveal if there had been a death in the house. “I was looking for a Carfax for homes,” he said, referring to the service that supplies vehicle history reports to used-car buyers. “Instead, I found pages and pages of Google search results asking the question ‘How do I find out if my house is haunted?’ ”

Two years ago, Condrey launched DiedInHouse, a website that allows customers to find out if someone died at a specific address and the cause of death. Using news reports, obituaries, and some search juju Condrey prefers not to disclose, his company has built a database of 4.5 million houses that were the sites of confirmed deaths.

DiedInHouse's reports for a single address start at $11.99, and customers use the service for everything from due diligence before buying a new home to investigating reports of supernatural activity. Mortgage brokers are taking the occasional peek as well.

Dave Schrader, a paranormal investigator, says he uses the site to scout locations for the Travel Channel show Ghost Adventures. Customers have paid for 30,000 property searches so far—enough to allow Condrey, a 44-year-old IT manager by day, to hire a part-time staff for the website.

Setting aside ghost stories, there are some down-to-earth forces fueling the business. Stigmatized properties, which is a polite way of describing the scene of a horrible crime, often sell at a 10 percent to 15 percent discount shortly after the event, said Randall Bell, a Laguna Beach-based appraiser who specializes in real estate damages.

In extreme cases, a grisly crime can make a house virtually unsellable. The infamous Los Angeles mansion in Loz Feliz where a doctor killed his wife and then himself in 1959 has remained abandoned since the early 1960s but could be worth $2 million, according to crime journalist Jeff Maysh. Some real estate investors see an opportunity for a windfall profit. The Guilderland (N.Y.) house where a family of four was murdered last October was sold for $30,000, then quickly relistedfor $199,000 as the “perfect place for a growing family to plant their roots.”

Even though it's common for homebuyers to want to know whether a violent crime occurred at their dream house, many states have lax laws regarding what sellers have to disclose. In Pennsylvania, for instance, the state supreme court ruled last year that "psychological stigmas" such as deaths don't need to be disclosed at all (unlike structural damage). It's the same in Massachusetts, where state legislation also allows sellers to keep quiet about "alleged para psychological or supernatural phenomenon." That's in contrast to California, where sellers have to tell buyers about most types of deaths that occurred at the property in the last three years. South Dakota and Alaska require disclosure of murders or suicides in the last 12 months.

Condrey wants to expand the business and is adding data on property taxes and local school districts. He's also testing a service to sell leads to investors and real estate agents looking to score a bargain on properties priced by heirs to sell after the homeowner dies.

Still, Condrey seems more comfortable as an advocate for homebuyers who are afraid of ghosts. He uses his Facebook page to chide sellers for offloading homes on unsuspecting buyers. In one example, a home where a woman allegedly killed her four daughters she thought were possessed by demons in 2009 was sold for $40,000 after fetching $139,000 a year earlier. In 2010, it sold again for $132,000. “I wonder,” Condrey wrote in an e-mail, “if the new owner was made aware.”